When people talk about spam, they generally mean 2 things: high-volume cheap spam from lots of empty accounts, or a low-volume aggressive marketing from well funded non-empty accounts. The first one (it's DoS essentially) can only be properly prevented by LN paywall. The second one (let's just call it 'noise') - can be prevented by reputation filters, of which I believe TrustRank to be the proper one. 

We could use TrustRank even now to filter the first type of spam (I do at nostr.band), but then new people joining nostr would not become visible in a global feed without getting some friends to follow and interact with them (without gaining some reputation). LN is better because it filters out high-volume spammers without hurting the normal noobs.  

 i like to define spam as content i dont want 

 Nostr definitely has a spam problem. When I was gathering events for the Nostrum Search database, out of ~2.5 million events about 1.2 million were pure spam that had to be filtered out (mostly notes that didn't use any of canonical note "kinds"). I'm sure these issues will be tackled over time, but I agree that a pay-to-play LN solution is probably the best one. 

 Agreed. Although non-canonical kinds is just people experimenting with other uses of the protocol - irrelevant kinds don't influence the popular nostr clients focused on kind:1 posts. It might be a problem for relays, but then they could just ban useless kinds, so it's not a big issue. 

 Nostream supports whitelisting event kinds which should help 

 Referral spam can also be annoying, with lots of plebs falling for it until they learn not to do it. Often referral spam is done because people genuinely like a product. And then there is the stupid chain mail stuff - help this kid in need by re-sharing this sob story ...
 

 Agree, although I feel muting those on the client is a bit simpler basis because you could just add most of that stuff into some word stop list.